This worksheet is intended to function as a checklist of things to consider
when researching a PLC. It is broken down into the most important areas to consider
when choosing a PLC. Following are guidelines for completing the checklist:

1) Determine whether your system is new or existing: Will your system
be installed from scratch or are there existing products already
installed? The rest of your system will need to be compatible with
new components.

Why this is important: Certain controller products may not be
compatible with others. Making sure your existing products are
compatible with any new products you are researching will save
you time and money.

Why this is important: Certain environments may affect the operation
of a controller. For example, typical controllers have an operating
temperature of 0-55 degrees Celsius (32-130 degrees F). If
your application will include any extreme environmental conditions,
or you have specific codes at your facility that must be met,
you will need to either research products that meet those specifications
or design the installation to meet requirements.

3) Determine how many discrete and analog devices your system will have:
How many discrete and analog devices will you have? Which types (AC, DC, etc.)
are needed?

Why this is important: The number and type of devices your system
will include is directly linked to the amount of I/O that will be
necessary for your system. You will need to choose a controller that
supports your I/O count requirements and has modules that
support your signal types.

4) Determine whether your system will require any specialty features:
Will your application require high-speed counting or positioning?
What about a real-time clock or other specialty feature?

Why this is important: Specialty functions are not necessarily available
in a controller CPU or in standard I/O modules.
Understanding the special functions your system may perform will
help you determine whether or not you will need to purchase additional
specialty modules.

5) Determine the type of CPU you will need: How much memory will your
system require? How many devices will your system have (determines data
memory)? How large is your program, and what types of instructions will your
program include (determines program memory)? How fast a scan time do
you need?

Why this is important: Data memory refers to the amount of memory needed
for dynamic data manipulation and storage in the system. For example,
counter and timer instructions typically use data memory to store setpoints,
current values, and other internal flags. If the application requires historical
data retention, such as measured device values over a long period of time,
the size of the data tables required may determine the CPU model you
choose. Program memory is the amount of memory needed to store the
sequence of program instructions that have been selected to perform the
application. Each type of instruction requires a specific amount of program
memory, typically defined in a programming manual. Applications that are
basically sequential in nature can rely on the I/O device rule of thumb to estimate
program memory (five words of memory for each I/O device); complex
applications will be more difficult to judge.
If scan time is important in your application, consider the CPU processor
speed as well as instruction execution speed. Some CPUs are faster at
boolean logic but slower with data handling instructions.
If special functions such as PID are required, the CPU you select may make
those functions easier to perform.
For program memory required, follow this rule of thumb: 5 words of program
memory for each discrete device and 25 words for each analog
device.

6) Determine where your I/O will be located: Will your system require
only local I/O, or both local and remote I/O locations?

Why this is important: If subsystems will be needed at long distances from
the CPU, you will need a controller that supports remote I/O. You will also
have to determine if the remote distances and speeds supported will be
adequate for your application. Serial and Ethernet-based I/O hardware
are two typical choices available for most systems. This I/O may also be
referred to as distributed I/O, and may require a particular protocol, such
as Modbus.

7) Determine your communication requirements: Will your system
be communicating to other networks, systems, or field devices?

Why this is important: Communication ports (other than the programming
port) are not always included with a controller. Knowing your system
communication requirements will help you choose a CPU that supports
your communication requirements, or additional communication modules
if necessary.

Why this is important: Certain controllers may not support every type of instruction.
You will need to choose a model that supports all instructions that you
may need for a specific application. For example, built-in PID functions are
much easier to use than writing your own code to perform closed-loop
process control. Typical instructions such as timers, counters, etc. are available
in most controllers; note any other special instructions required here.

What now?

Once you have recorded the information on the worksheet and determined your
requirements, use this sheet to find a PLC that meets your requirements. With
your requirements outlined, it will be much simpler to find a product with the
necessary number of I/O points, features, memory, etc. that your application
requires.